I'm interested in how people consider varnish on a valve cover to be "baked on".
The varnish isn't formed on the valve cover, it is transported there by oil that is at it's saturation point with varnish, and cools below the temperature that it drops out in those dark brown striations seen across rocker covers, where the chain goes etc.
To get rid of it, you need to get the varnish back into solution, transport it away, and get rid of it...and the only place that it can go is into the drain pan. Once new oil, or a new additive has sucked up as much as it can hold, then any new varnish that is formed, gets thrown at the same old places.
It took a few hundred thousand miles, and many many gallons of varnish saturated oil to make a deposit, it will take many many oil changes to make it go away. Each and every one of those changes must be "early", to allow the new oil to suck up the varnish that's already there.